Abstract C. I. Acid Black 234 and C. I. Mordant Black 9 were decomposed in water using ferrioxlate and H 2O 2 during sunny or cloudy days, respectively. Some effecting factors such as Fe(III) ion, oxalate ion, H 2O 2 and solar irradiation intensity were investigated. The decomposition of the two dyes was evaluated with respect to decoloration percentage or reaction rate constant. Also, their degradation and mineralization under different solar irradiation were examined by UV–vis spectrum and TOC measurements. The results indicate that the effects of concentration of Fe(III), oxalate and H 2O 2 on dye decomposition are dependent significantly upon solar illumination. Increasing irradiation intensity caused the proportional enhancements in decoloration rate constant. We also found it more difficult to decompose C. I. Acid Black 234 under two weather conditions, comparing with C. I. Mordant Black 9. In addition, the degradation and mineralization of the two dyes was reduced by low irradiation.